1. Field of the Invention
The present invention relates to a heat-sensitive recording material which has both a superior image contrast and a high stability of developed image against oily substances and solvents such as alcohols.
2. Prior Art
A heat-sensitive recording sheet that utilizes a heat color-forming reaction occurring between a colorless or pale-colored chromogenic dyestuff and a phenolic material, or an organic acid is disclosed for example, in the Japanese Patent Publication Nos. 4160/1968 and 14039/1970 and in the Japanese Laid-Open Patent Application No. 27736/1973, and is now widely applied for practical use.
In general, a heat-sensitive recording sheet is produced by applying on a support, such as paper, film etc., the coating which is prepared by individually grinding and dispersing a colorless chromogenic dyestuff and a color-developing material into fine particles, mixing the resultant dispersion with each other and then adding thereto binder, filler, sensitizer, slipping agent and other auxiliaries. The coating, when heated, undergoes instantaneously a chemical reaction which forms a color. In this case, various bright colors may be formed depending upon the selection of colorless chromogenic dyestuff.
These heat-sensitive recording sheets have now been finding a wide range of applications, including medical or industrial measurement recording instruments, terminal printers of computer and information communication systems, facsimile equipments, printers of electronic calculators, automatic ticket vending machines, and so on.
In recent years, as the heat-sensitive recording systems are widely used and the applications of such rcording are diversified, high image density is now required for the improvement of the resolution. The heat energy of the thermal head in the recording equipments capable of such high density is more minimized. Therefore, it is required that the heat-sensitive recording sheet has a higher color-forming sensitivity sufficient for producing clear chromogenic record with such small heat energy.
Meanwhile, the heat-sensitive recording sheets are inevitably touched with the hand of man, in view of their function as recording sheets of the information.
As the fingers of the operator are often adhered by solvents such as alcohols etc., or by oily substances such as his hair tonic daily used and oils contained in the sweat on his skin, it may be said that the heat-sensitive recording sheets are most frequently contaminated by such substances. In general, the heat-sensitive recording sheets have insufficient stability against these oily substances and the solvents such as alcohols, acetone etc., so that the density of the developed color image on the contaminated part is often reduced or disappeared. The contaminated white ground causes the phenomenon of discoloration or color forming. Their reasons cannot be sufficiently elucidated yet, but it is supposed that such substances partly dissolve the coloring layer consisting of the fine granular basic colorless dyestuff and organic developer or coloring reactant thereof, or make the coloring layer or coloring reactant thereof unstable.
Also, the developed images disappear, or the coloring reaction through the solvent between dyestuff and color-developing agent, i.e. the color development of the ground color, occurs.
In order to increase these stabilities, there was proposed a method in which a barrier layer is formed on the color-developing layer comprising both a leuco dyestuff and an organic color-developing agent to prevent the contact with such solvents or oily substances. However, this method has disadvantages that a barrier layer with good oil-resistance and solvent-resistance is not obtained and the lowering of the sensitivity occurs.
Besides the heat-sensitive color-developing system in which the above colorless dyestuff is used, a color-developing system under the use of metal compounds is known.
For examples, the Japanese Patent Publication No. 8787/1957 describes the combined use of iron stearate (electron acceptor) with tannic acid or gallic acid, and the Japanese Patent Publication No. 6485/1959 describes the combined use of an electron acceptor such as silver stearate, iron stearate, gold stearate, copper stearate or mercury stearate with an electron donator such as methyl gallate, ethyl gallate, propyl gallate, butyl gallate or dodecyl gallate. Since these heat-sensitive recording sheets are based on a heat-sensitive copying system by means of the heat energy of light, they bring the troubles of accumulated residues and sticking under applying to heat-sensitive recording system which uses the thermal printing heads. In this case, they have as disadvantages a low image density, greenish color, poor brightness of the background, inferior stability against solvents such as alcohols, and the flowing-out of the color-developing layer.
Further, the Japanese Laid-Open Patent Application No. 89193/1984 describes a combination of a color-developing system using a leuco dyestuff and a color-developing agent and of a color-developing system using a metal compound of higher fatty acid ferric salt and polyvalent phenol. However, such combination is disadvantageous in costs, since it requires a protecting layer for hiding colored parts. Further, it has a defect that solvents such as alcohols are penetrated through the pin holes of a protecting layer, resulting in coloring (contamination) caused by a reaction between a leuco dyestuff and a color-developing agent which are present in a color-developing layer.